Polymer resins, such as polyethylene terephthalate (PET), are widely used in the packaging industry. PET is a linear, thermoplastic polyester resin. The myriad advantages of PET include toughness, clarity, good barrier properties, lightweight, design flexibility, chemical resistance and good shelf-life performance. Furthermore, PET is environmentally friendly since it can often be recycled. These characteristics of PET make it a popular material in the manufacturing of containers, for example, beverage bottles.
There are a variety of production methodologies to produce PET containers. For example, injection stretch blow molding (ISBM) is commonly used to make PET bottles. Of the various methodologies, one-piece PET containers having an integrated handle (handleware) are commonly formed using extrusion blow molding (EBM). The EBM process includes extruding a polymer resin in a softened state through an annular die to form a molten hollow tube or parison. The molten parison is placed in a hollow blow mold having a cavity corresponding to the desired shape of the container being formed. Air is injected to inflate the parison against the interior walls of the blow mold. Upon contact with the walls, the parison cools rapidly and assumes the shape of the mold.
Polyesters are typically classified by inherent viscosity (I.V.) as a measure of molecular weight. To form beverage bottles, “bottle grade” PET having an I.V. of about 0.72-0.84 dl/g, is typically used. Bottle grade PET has linear polymer chains and by design has a melt viscosity that is low enough to enable a faster injection stretch blow molding step with the least resistance to flow. Bottle grade PETs generally cannot be used in the production of larger handleware containers using EBM because of low melt strength. Melt strength is quantified by measuring melt viscosity at very low shear rates (approaching zero shear rate). Low melt strength hinders the ability to form a suitable parison. If a parison in the molten state has insufficient melt strength, during the EBM process, as the parison is drawn down by its own weight, the parison forms an hour-glass shape or may completely collapse, thereby resulting in the inability to produce a container. As melt strength increases, material distribution in the walls of the resultant container improves, and the process becomes more controllable and repeatable.
To make PET suitable for EBM, high molecular weight PET having an I.V. of 1.0 dl/g or greater as measured by solution viscosity, could be used. For PET resins I.V. is used as a measure of molecular weight. The average molecular weight of a resin reflects the average length of polymer chains present therein. In general, melt strength increases with chain length and, thereby, also increases with molecular weight. However, higher I.V. polymers generally require higher processing temperatures. Higher temperatures may cause the resin to thermally degrade, resulting in more yellowness in containers produced. Moreover, the process window for a high I.V. PET in an EBM process narrows, making it difficult to run a stable extrusion blow molding operation over an extended period. In addition, longer chain lengths are more susceptible to shear and thermal degradation. Higher I.V. resins also tend to be more expensive than bottle grade PET resin commonly used to produce containers, increasing manufacturing cost.
An alternate solution to achieving a desirable melt strength is to use branched PET copolymers. An example is the Eastar Copolyester EB062, manufactured and marketed by Eastman Chemical Company. EB062 is a lightly branched PET copolymer having an I.V. of 0.75 dl/g. Branching effectively increases the melt strength of the resin. The EB062 copolymer also suppresses crystallization, which enables containers to be produced with high clarity, while allowing the resin to be processed at lower temperatures. Lower processing temperatures result in higher melt viscosity which in turn serves to improve process stability in extrusion blow molding.
While these characteristics serve to produce a container having good aesthetics and consumer appeal, they present challenges in terms of PET recyclability. High levels of such copolymers suppress the rate and extent of crystallization to such a level that results in a slow crystallizing resin. Amorphous or such slow crystallizing resins, when added to the PET recycling stream, tend to cause sticking, agglomeration and bridging issues during the drying process. This characteristic is a major impediment to PET recycling and, as a result, makes such PET resins unsuitable for reuse in the PET recycling process. When bottle grade PET and amorphous and/or slow crystallizing PET are combined, the performance of the molten blend of resins may exhibit a reduced rate and extent of crystallization, insufficient melt temperature, and insufficient physical properties such as hardness, tensile and flexural properties. The severity of these undesirable effects typically bears a direct relationship to the percentage of amorphous or slow crystallizing PET content in such melt processed recycled PET. As a result, PET copolymers such as EB062 generally are not recyclable when the concentration in the PET recycle stream exceeds 5% by weight blended with ground-up PET bottle flake.
There remains a need for polyester compositions that are suitable for extrusion blow molding and which are recyclable in PET recycling streams.